This invention relates to a dropout detection device for a device for playing back optical type discs such as Compact Disc in the Compact Disc Digital Audio System standard and a video disc according to the Laser Vision standard. The invention is directed to a dropout detection device capable of detecting a dropout in a reproduced signal with a simple construction.
A dropout in a reproduced signal from an optical type disc is produced due to various causes such as mixing of dust or forming of a pin hole in the material of the disc during manufacture of the disc and dirt or scratch on the surface of the disc. The dropout causes noise in a reproduced picture or sound. It is therefore a general practice to detect a dropout in a reproduced signal and substitute the section in which the dropout has occurred by a substitution signal.
In a prior art dropout detection device, detection of a dropout is made by examining the level of an output signal and judging that a dropout has occurred when there has been an abnormally large change in the level of the output signal.
An example of a video disc playback device including such prior art dropout detection device is shown in FIG. 2. A general structure of this video disc playback device is disclosed in, for example, a Japanese magazine entitled "Radio Technique", January 1986, Pages 164-168 (FIG. 1) and the prior art dropout detection device is disclosed in, for example, Japanese Patent Preliminary Publication No. 51-8822 as a prior art.
Referring to FIG. 2, a signal recorded on a disc 10 is read by an optical pickup 12. In the optical pickup 12, laser beam reflected by the disc 10 is received by a four-split photodiode 14 and tracking photo-diodes 16 and 18 through a cylindrical lens. The four-split photodiode 14 adds detected outputs on each diagonal among four detected outputs A, B, C and D together and thereupon produces signals A+C and B+D. The tracking photo-diodes 16 and 18 provide signals E and F respectively.
The signals A+C and B+D are subject to subtraction between themselves by a subtractor 20 for detecting a focus error. A focus servo circuit 22 drives a focus coil 24 so that the focus error will be reduced to zero.
The signals E and F are subject to subtraction between themselves by a subtractor 26 for detecting a tracking error. A tracking servo circuit 28 drives a tracking coil 30 so that the tracking error will be reduced to zero.
The signals A+C and B+D are added together by an adder 32. The sum signal A+B+C+D is amplified by an HF amplifier 34, is subject to FM detection by an FM detection circuit 36 and thereafter is applied to a synchronizing signal detection circuit 38 for detection of a horizontal synchronizing signal. A disc servo circuit 40 compares a detected horizontal synchronizing signal with a reference clock from a reference signal generator 42 in frequency and phase and controls a disc motor 44 so that these signals coincide with each other.
A main TBC (Time Base Collector) 46 removes jitter (sway on time axis) contained in a reproduced signal from the disc and this circuit 46 is constructed in the form of a delay circuit for continuously delaying a binary signal. As such delay circuit, a circuit utilizing the signal delaying chracteristics of a CMOS gate as described in the U.S. Patent Application No. 760,332 for example is employed. A TBC control circuit 48 compares, in phase, a horizontal synchronizing signal detected by passing the output signal of the main TBC 46 through an FM detection circuit 150 and a synchronizing signal detection circuit 152 with a reference clock corresponding to a normal horizontal synchronizing signal and controls delay time of the main TBC 46 in accordance with a phase error between these signals for absorbing the jitter.
An output signal of the main TBC 46 is passed through a band-pass filter 50, an FM detection circuit 52 and a low-pass filter 54 and a left channel audio signal thereby is produced. The output signal of the main TBC 46 is passed also through a band-pass filter 56, an FM detection circuit 58 and a low-pass filter 60 and a right channel audio signal thereby is produced.
The output signal of the main TBC 46 is applied also to a color TBC 64 through a band-pass filter 62. The color TBC 64 is a circuit provided for absorbing minute jitter which has not been absorbed by the main TBC 46. The color TBC 64 is constructed in the form of, for example, a delay circuit for continuously delaying a binary signal by utilizing a CMOS gate circuit as in the case of the main TBC 64. The TBC control circuit 48 compares, in phase, a color burst obtained by passing the output signal of the color TBC 64 through an FM detection circuit 66 and a color subcarrier detection circuit 68 with a reference clock of 3.58 MHz corresponding to a normal color subcarrier and controls delay time of the color TBC 64 in accordance with a phase error between these signals for absorbing the minute jitter contained in the reproduced signal.
The output signal of the color TBC 64 is applied to the FM detection circuit 66 for FM detection and thereafter is provided as a video signal from a terminal a of a switch 70.
A dropout detection circuit 72 detects a dropout in a reproduced signal and comprises an FM detection circuit 74 and a level comparator (window comparator) 76. The output of the band-pass filter 62 is FM detected by the FM detection circuit 74 and its level is detected by the level comparator 76. If there is a dropout in the reproduced signal from the disc, a noise of a large level as shown in FIG. 3 is produced when the video FM signal is FM-detected. Accordingly, by comparing the output of the FM detection circuit 74 with suitably established upper and lower detection levels of the level comparator 76, the dropout can be detected.
If a dropout has been detected, the switch 70 is switched to a terminal b side by a control circuit 78 during one horizontal scanning period 1H. Since a signal which has been delayed for 1H by a 1H delay circuit 80 and FM-detected by an FM detection circuit 82 is provided to the b terminal side, a signal which has interpolated a section of 1H in which the dropout has occurred with the preceding 1H section is provided by the switch 70 whereby correction of the dropout is achieved.
Since this dropout detection circuit 72 detects the dropout by watching an amplitude modulation component in a reproduced signal, the dropout cannot be detected until an FM video signal has been FM-detected by the FM detection circuit 74. Accordingly, this dropout detection circuit 72 necessitates a rather complicated circuit construction.
It is, therefore, an object of the invention to provide a dropout detection circuit for an optical type disc playback device capable of detecting a dropout from an original reproduced signal without subjecting it to the FM detection.